1. Field of the Invention
The present invention concerns a method for determination or adaptation (i.e., setting) of a shim for homogenization of a magnetic field of a magnetic resonance apparatus, the magnetic field being provided for the generation of magnetic resonance exposures of a specific examination region, and an associated magnetic resonance apparatus.
2. Description of the Prior Art
The magnetic field in a magnetic resonance tomography apparatus in all cases initially exhibits disruptions of the homogeneity. The reasons for this are due not only to the manner by which the field is produced but also due environmental conditions arising from the image acquisition or existing in the examination space. Disruptions also may be due to the examination subject.
In order to compensate for such inhomogeneities of the magnetic field, components known as shims are used which are coils or steel pieces and the like that are placed in the magnetic field or in proximity to the magnet in order to influence the generated magnetic field in a desired manner.
A shim iteration is implemented in order to determine an optimal shim for a specific acquisition sequence. As a consequence of this iteration, spherical functions are calculated that specify the suitable shim in the first order, as what are known as gradient offsets, and in the second order, as what are known as “advanced” shims, for an examination or acquisition region in question. The calculated functions thus specify the basis for a compensation of magnetic field inhomogeneities.
In order to select the shim volume (thus the region for which a shim iteration or calculation should be implemented), or to precisely adapt the shim volume for the further course of the examination, for calculation of the shim it has been typical to use a rectangular or cuboid volume that includes the slice exposures or groups of slice exposures that are to be generated. This cuboid is selected as an adaptation shim volume (adjust shim volume).
For experienced operators or users of the relevant software, it is possible to redefine the position or extent of the cuboid adaptation volume, so independence of the position and orientation of the cuboid from the slice groups can be achieved.
Nevertheless, the shim volume (thus the volume on the basis of which the optimal shim is determined for an image exposure to be implemented) has previously been limited to a cuboid three-dimensional or a rectangular region. Difficulties result form this in the case of various image acquisitions, for example in cardiology in which image exposures of the heart must be acquired, whose shape significantly deviates from a cuboid. If a cuboid is now simply placed around the heart in order to determine or to optimally adapt the shim, this means that, for example, regions in which heart tissue is not located (namely surrounding air or other tissue) must be included in the calculation of the spherical functions. The calculation is made more difficult by such transitions.
Similar problems result in the acquisition of other examination regions such as, for example, acquisitions of the liver in which a rectangular shape or a cuboid is likewise only marginally suitable to acquire the essentially ellipsoid shape of the liver. The brain as well as the spinal column also exhibit three-dimensional forms that clearly deviate from a cuboid shape.
A method for determination of design parameters for magnet structures for generation of magnetic fields in predetermined spatial volumes as well as a method for correction of magnetic fields in predetermined spatial volumes, in particular for magnetic resonance apparatuses, are known from EP 1 705 494 A1. The predetermined spatial volumes, in which a static magnetic field should be generated that can be corrected as needed in the framework of shimming exhibit an ellipsoid shape.